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An anonymous reader writes "The commercial cargo ship Dragon left the International Space Station, and is heading home with nearly two tons of science experiments and old equipment. From the article: 'The unpiloted Dragon departed the International Space Station at 9:26 a.m. EDT to begin a trip expected to culminate just after 3 p.m. with a parachute-assisted splashdown in the Pacific Ocean, about 300 miles west of Baja California. NASA astronaut and station commander Steve Swanson controlled a 58-foot robotic arm that pulled the Dragon from its Harmony node port at 8 a.m., then released the capsule into space 266 miles over the ocean south of Australia.'"

quote>Why would they use a measure of WEIGHT instead of a measure of MASS?

Ton is already ambiguous, but since it is a USA media article, its safe to assume that they meant what is also known as the short ton, or 2000 pounds. The pound is defined as 0.45359237 kg, so it is, by definition, a unit of mass.

A pound is a unit of weight and can correspond to any kg mass, determined by the gravity of the place where it is being measured.
Weight is dependent on gravity, mass is not. Welcome to 5th grade science class

The pound or pound-mass (abbreviations: lb, lbm, lbm, [1]) is a unit of mass used in the imperial, United States customary and other systems of measurement. A number of different definitions have been used, the most common today being the international avoirdupois pound which is legally defined as exactly 0.45359237 kilograms, and which is divided into 16 avoirdupois ounces.

The real world is not always as simple as what you learned in 5th grade science, when your teacher said "The pound is a unit of weight, not mass", he was correct and incorrect at the same time due to the ambiguous nature of the unit.

It is a mass that accelerates by 1 ft/s2 when a force of one pound-force (lbF) is exerted on it. One slug has a mass of 32.174049 lbm or 14.593903 kg based on standard gravity, the international foot, and the avoirdupois pound

So to avoid the confusion between lbm and lbf, you make a new unit of mass that converts to the other unit of mass with the easy to remember value of 32.174049! Brilliant!

The word "pound" can refer to mass, weight, or currency. To disambiguate, terms such as "pound-force", "pound-mass" and "Pound sterling" can be used instead, but otherwise the meaning is often clear in context. In particular, the ton is defined in terms of the pound-mass (2000lb or 2240lb, depending on who you ask), although officially the various "ton" units are defined in terms of the kilogram - also a unit of mass. And of course, the metric ton is 1000kg.

The term "ton" is widely used by many cultures and is a hold-over from earlier mass-weight units. If you want to call it a megagram, that is your call, but the widespread and common usage is still "metric ton" by both governments and especially industrial users. If you want to put on blindfolds and pretend that such units don't exist, that is your own problem and you will be laughed out of many conferences by being such a stupid stick in the mud about such silly things.

So people who drive cars, and therefore use public transportation less or not at all, should pay more so that people who do use the system pay less?

More likely someone doesn't know that there is a difference between a Ton and a Tonne, and figures they're just the british and american ways of spelling the same thing. The dragon capsule is rated to return up to 2500kg of mass to the earth, so it stands to reason that this is just a lack of pedantry on the part of the author.

Pounds are both a measure of weight and mass, and the USA Today article uses pounds (not tons, Slashdot did that conversion) because, for better or worse, the US population is more familiar with US customary units than metric units, and USA Today is marketed at a US audience (the name is a bit of a clue). NASA also uses US units for some mind-baffling reason (maybe it likes destroying Mars Orbiter [wikipedia.org] missions?) so the US units make sense in this story.

People landed on the Moon with pounds, feet, gallons, and other "Imperial" measurement units. It is possible to do these things and not need the Metric system. Hell, they landed on the Moon using slide rules which performed most of the calculations and for those things that needed faster computations, NASA needed to invent a real-time operating system (something that didn't even exist prior to NASA's use of the OS).

Don't get me started on how silly the metric system is too. It has its use and is widely u

The problem with metric is that some of the arbitrary base units are more difficult for humans to estimate and use. For example, let us take some common units of measure: centimetres are too small, decimetres are too large. Both are inferior for human estimation compared to inches. An inch can be approximated more easily using say a segment of a finger. Even feet are easier for a lot of people to estimate than metres (or yards). Especially for in-between distances that are neither small nor large. In

The only real units IMHO are Plank units, but even those aren't useful for practical everyday applications.

BTW, you don't need to worry about unit conversions unless you are using different subsystems from different countries or manufacturers that aren't using the same units. It really doesn't matter what you are using even if it is centifurlonghs per microfortnights for velocity. While done as more or less a practical joke, the microfortnight was used by VMS in its timing circuits and used for various ac

I don't find metric any more difficult to estimate or use. I use both metric and imperial units every day, and your message is the first time I've ever seen anyone saying a centimeter is hard to estimate. I find estimating stuff in cm no more difficult than doing the same in inches.

Where metric wins *massively* is when you're having to deal with using lots of different types of measures, such as estimating things such as power requirements, energy requirements and that kind of thing because the units are de

So NASA spent $1.6 billion for the CRS program, that is for 12 missions [1] [wikipedia.org]. That is $75 million for mission. The payload of the CRS-3 mission, the biggest so far by the way, was 4,605 pounds (the declared maximum is 7,300 lb)[2] [nasaspaceflight.com], in other words $16,200 for pound of payload, including packaging. I'd like to know how does that compare to other space transport services.

The COTS program never even required hauling cargo back from orbit. The contract was strictly for sending stuff up to the ISS, not for returning anything (which is why the Cygnus spacecraft doesn't have re-entry capabilities). SpaceX threw the re-entry capabilities on as an extra and offered it to NASA,which NASA is certainly using.

That the Dragon is the only vehicle currently in production which is capable of returning more than a hundred kilograms of stuff from orbit (that is the Soyuz spacecraft, where

No, the extra cargo that can be returned other than some cosmonauts is about 100 kg. Most of the life support for the Soyuz is also in the "orbital module" part that is left behind in space... part of what gives both the Shenzhou and Soyuz spacecraft its very distinctive shape and how it looks very different from the Orion/Apollo/Dragon capsules that don't have separate parts.

Keep in mind that the Soyuz and Shenzhou spacecraft have two parts: the re-entry capsule and the life support "orbital" module that

The disposable service module on the Dragon is not habitable and in fact can't be serviced from inside of the capsule. About the only thing it really has is just solar panels... not even RCS fuel tanks or thrusters.

The Shenzhou and Soyuz both have habitable modules that contain food, life support, and even scientific experimental hardware (when applicable) that astronauts use when in orbit. It is a bit different than what the Dragon has.

The space shuttle was $450 million per mission not including development costs. That would lift 24 ton and a lot of volume to ISS. That was good for building the space station but perhaps overkill for the maintenance. They are not even using the full capability of the Dragon spacecraft.

The Dragon will only move 3.3 ton to the ISS. If you only count weight by dollar this is more expensive than a Space Shuttle launch. On the other hand you will get much more frequent deliveries which may be what is needed now.

If you count development costs, each Space Shuttle launch was 1.5 billion USD. Viewed this way, the CRS program for Space X is just one shuttle. And perhaps this is the correct way to do the accounting considering that the 1.6 billion that Space X receives also has to cover their development costs. I would expect that they can give a good discount on future launches, should NASA want more than 12.

Better than just discounting once development is paid off, actually... part of that R&D investment is into making the first stage, the Falcon 9 booster, re-usable. Currently they are single-use and amount to 70% of SpaceX's costs per launch. A reusable first stage would let SpaceX cut their costs by a tremendous margin.

It's really astonishing how much SpaceX is achieving with the budget they have. The space shuttle may have been a technological marvel in terms of capabilities, but it was unreliable, exp

Should be noted that the capsule was physically full. It could have carried more mass but the average density of the cargo isn't that high so it would seem that it could've taken more up while in fact it took a full load.

When resupplying the ISS, it is not all about up/downmass. Physical dimensions also matter and some cargo is lighter than others.

Most of the early flights of the Dragon (including this CRS-3 flight) have been flying low value cargo to the ISS. Mainly food and consumables (spare parts, batteries, a few laptops, and other small stuff) that if it was lost wouldn't necessarily be all that important. As you've mentioned, this is also somewhat bulky, but there has been spare room left over. That is why one of the surprises that SpaceX sent up was some ice cream (a very rare treat in space) and a few bags of snack food that wasn't on th

Well, you can simply use Google to figure out things. However, I was curious, so I did some of this leg work.
The Soyuz-ST rocket that launches 7.8 tonnes to LEO [wikipedia.org] costs
$61M in 2006. That was just the launch vehicle. [google.com]
Now, that was just the launch, and that was almost 10 years ago. So, the price is at least 50% more (russian money has appreciated).
In addition, that does not include the costs of the progress itself. That is likely going to double the price. IOW, you are probably looking at 180M or so for